tst_qfilesystemmodel: increased test's permitted runtime

This test has recently timed out a few times on OS X test runs, with
no relevant changes to account for the timeout.

Task-number: QTBUG-27890
Change-Id: Ia24f7812ed2a0b3eac51847a7dacbc9f225b48b8
Reviewed-by: Simo Fält <simo.falt@digia.com>
Reviewed-by: Sergio Ahumada <sergio.ahumada@digia.com>